The present invention relates to a speed control apparatus for an elevator which is driven by a current control type inverter, and more particularly to attaining an improved ride at the time of the start of elevator.
In recent years, an A.C. variable-speed control in which a frequency converter is combined with a stout an inexpensive A.C. motor has been applied to elevators. Especially for elevators operating in a low speed region in which the primary voltage control of an induction motor has hitherto been performed, the A.C. variable-speed control has attracted notice from the viewpoint of energy conservation.
In order to achieve a control performance equal or superior to that of a D.C. motor by the use of the aforementioned control system, the A.C. motor needs, likewise to a D.C. motor, to have its primary current controlled separately and independently as to the component of the current contributive to a field (field component current i.sub.d) and the component orthogonal thereto and contributive to a torque (torque component current i.sub.q).
A conventional speed control system will be described with reference to FIGS. 4 and 5. In the figures, numeral 1 designates a speed command generator which generates a speed command signal .omega..sub.rR. Numeral 2 designates a control device receiving the speed command signal .omega..sub.rR and a speed detection signal .omega..sub.r produced by a speed detector 12 connected an induction motor 11 to determine primary current commands i.sub.uR, i.sub.vR and i.sub.wR for the motor 11. As shown in FIG. 4, the control device 2 comprises an adder 21 which evaluates the deviation between the speed command signal .omega..sub.rR and the speed detection signal .omega..sub.r, a speed calculating circuit 22 which calculates a torque command T.sub.R on the basis of the deviation signal, and a primary current command calculating circuit 23 which calculates the primary current commands i.sub.rR, i.sub.vR and i.sub.wR on the basis of the torque command T.sub.R and the speed detection signal .omega..sub.r.
Numerals 3-5 indicate adders which evaluate the deviations between the primary current commands i.sub.uR, i.sub.vR and i.sub.wR and the actual currents i.sub.u, i.sub.v and i.sub.w of the motor detected by current detectors 8-10, respectively. The deviation signals are applied to a current control circuit 6, which controls a frequency converter 7 on the basis of the deviation signals so that the primary currents i.sub.u, i.sub.v and i.sub.w may agree with the respective current commands i.sub.uR, i.sub.vR and i.sub.wR. Thus, output currents from the frequency converter 7 are controlled to feed the induction motor 11 with predetermined A.C. power. For a complete elevator operating arrangement, a sheave, 13 a rope 14, a cage 15, and a counterweight 16 are also illustrated.
Here, the primary current command calculating circuit 23 of the control device 2 has an arrangement in FIG. 5. In the figure, symbols 23a, 23b and 23c denote multipliers, symbol 23d an adder, symbol 23e a sinusoidal wave generator which delivers cos .theta., sin .theta. and cos (.theta.-2/3.pi.), sin (.theta.-2/3.pi.) components on the basis of an output from the adder 23d, and symbol 23f a two-phase/three-phase converter which calculates and delivers the primary current commands i.sub.uR, i.sub.vR and i.sub.wR on the basis of a torque component current command value i.sub.qR, a magnetic flux component current command value i.sub.dR and the outputs of the sinusoidal wave generator 23e. These constituent elements perform calculations in accordance with the following equations:
Now, letting .phi..sub.2R denote a secondary flux command value ("R" is affixed to command values), the torque component current command value i.sub.qR is given by: ##EQU1## L.sub.2 : secondary inductance of the motor, M: mutual inductance of the motor,
P: number of pole pairs.
And the magnetic flux component current command value i.sub.dR is given by: ##EQU2## (R.sub.2 : secondary resistance of the motor). As apparent from Eq. (2), a magnetic flux .phi..sub.2 within the motor becomes: ##EQU3## so that the secondary magnetic flux follows up a magnetic flux component current with a first-order lag.
In addition, a slip angular frequency command .omega.sR becomes: ##EQU4## The primary current commands i.sub.uR, i.sub.vR and i.sub.wR become: ##EQU5## since a control of constant magnetic flux is usually adopted in the control of an elevator, EQU .phi..sub.2R =constant (9)
with which Eq. (2) is reduced to: ##EQU6##
That is, the multiplier 23a calculates the torque component current command value i.sub.qR in accordance with Eq. (1), the multiplier 23b calculates the slip angular frequency command .omega..sub.sR in accordance with Eq. (4), and the two-phase/three-phase converter 23f calculate the primary currents i.sub.uR, i.sub.vR and i.sub.wR in accordance with Eqs. (5)-(7) respectively.
As understood from Eq. (3), the actual secondary flux of the motor follows up the d-axis current i.sub.d of the motor with the first-order lag. In the circuit of FIG. 5 arranged so as to give i.sub.dR in the form of .phi..sub.2R /M, therefore, the d-axis current is caused to flow before releasing a brake at the time of the start of movement of the elevator cage, for the purpose of establishing a field, whereupon when the field has reached a predetermined value, the brake is released so as to afford a desired linear torque, namely, in the form of ##EQU7## (This is called "pre-excitation".) The period of time for rendering the field the predetermined value by causing the d-axis current to flow beforehand (pre-excitation period) is ordinarily required to be 200-400 msec, and it is desirable from the viewpoints of an operating efficiency that the period of time be as short as possible.
In some high-grade elevators, a balancer for compensating the unbalanced load of the elevator is disposed in the cage 15, and a torque for compensating the unbalanced load is produced at the time of the start, whereby even when the brake is released, the cage does not start abruptly due to the unbalanced load.
In elevators of low speed, however, such a device is not used, and the cage starts abruptly after releasing the brake at the time of the start, so that a starting shock prevents a comfortable ride.